1. Field of the Invention
The invention relates in general to a structure of a semiconductor light emitting device, and more particularly, to a structure of a light emitting diode (LED) device, to a package structure of a flip-chip LED device, and to a light reflective structure being applicable for a LED.
2. Related Art of the Invention
In general, a light emitting diode (LED) constructed by an III-V semiconductor material can be provided as a wide bandgap light emitting device. The wavelength of the light emitted from the wide bandgap light emitting device ranges from infrared (IR) to ultraviolet (UV); therefore the entire spectrum of visible light is also covered. In recent years, due to the rapid development of the high illumination of the gallium nitride (GaN) LEDs having a blue/green light, the full-color LED display, white light LED and the LED for traffic signals are put into practice. Therefore, the application of a variety of LED also becomes more popular.
In principle, a fundamental structure of a LED device includes an epitaxy layer of a P-type and a N-type III-V group compound and a light emitting layer in-between. The light emitting efficiency of the LED device is dependent on the internal quantum efficiency of the light emitting layer and the light extraction efficiency of the device. A method of increasing the internal quantum efficiency includes, for the most part, improving the quality of the light emitting layer and the design of the structure. The method of increasing the light extraction efficiency includes, for the most part, decreasing the light loss caused by the absorption of the light emitted from the light emitting layer due to the reflection of the light inside the LED device.
In a conventional gallium nitride (GaN) LED device grown on the first substrate, such as sapphire, having an insulating property, since the positive and the negative electrodes of a gallium nitride (GaN) LED device are deposited on, in general, the same side of a first surface, and the positive electrode will screen out the emitted light from light emitting layer. Therefore, the packaging for a gallium nitride (GaN) LED normally uses the flip chip method. Thus, the emitted light will pass through the second surface. Moreover, a reflecting layer is formed on the topmost surface of GaN LED that faces the second substrate, in order to emit most of the emitted light towards the second surface of a GaN LED. Another advantage of using the flip-chip package process is, if a proper surface mount (so called surmount) substrate, for example, a silicon substrate is provided, the heat dissipation of the LED device is enhanced, especially under a high current operation. Accordingly, not only the light extraction efficiency is increased, the internal quantum efficiency of the light emitting layer will also be maintained.
Moreover, in order to improve the electrical property of the LED device, a semi-transparent nickel (Ni)/gold (Au) ohmic contact layer is first formed on the epitaxy layer surface, and a thermal process is performed to form adesirable ohmic contact, followed by forming a reflecting layer thereon. However, since the absorption of light of the Ni/Au layer is high (the transparency of that is about 60% to about 70%), and due to the thermal process, the interface between the epitaxy layer and the Ni/Au layer becomes too rough to reflect light. Accordingly, the light reflective efficiency of the bottom of the flip-chip LEDs device will be reduced.